While the Salesforce Winter ’21 Release Notes mention:
NOTE Email Template Builder isn’t available for previously created Classic and Lightning email templates.
What I did not expect is that it doesn’t matter so much that the template was created before the release, but that the creator of the template had been granted the Access Drag-and-Drop Content Builder System Permission (per Select Who Can Work in Email Template Builder) before creating the template.
How did I discover this? I created a template as a System Administrator and did not see the Edit in Builder button. I then created a permission set and added it to my profile. Still not there. I then spent a lot of time trying different browsers and turning the permission off and on again, all to no avail. Then I created another template and, voilà! there was the button.
I am either the only one dumb enough to perform the steps in this particular sequence or brave enough to admit it, as part of the time spent figuring it out was searching Trailblazers and https://salesforce.stackexchange.com for answers or even similar problems.
© Scott S. Nelson